1. Technical Field
The present invention relates to a multi-phase motor driving device used in an electrical power steering device of a vehicle, and in particular, to a technique of detecting the failure of the switching element.
2. Related Art
In the electrical power steering device of the vehicle, a multi-phase motor such as a three-phase brushless motor is arranged to provide a steering auxiliary force corresponding to the steering torque of the handle to a steering mechanism. The device for driving the multi-phase motor includes a control unit, a PWM (Pulse Width Modulation) circuit, and an inverter circuit. The control unit calculates the target value of the current to be flowed to the motor according to the steering torque detected by a torque sensor, and calculates a command value provided to the PWM circuit based on a deviation of the target value and the value of the current actually flowing to the motor. The PWM circuit generates a PWM signal having a duty ratio corresponding to the command value, and provides the same to the inverter circuit. The inverter circuit has a pair of upper and lower switching elements, which are turned ON/OFF according to the duty ratio of the PWM signal, for each phase, outputs the voltage of each phase corresponding to the duty ratio based on the ON/OFF operation of the switching elements, and drives the motor with such voltage. The current of each phase of the motor is detected by measuring the voltage at both ends of a phase current detection shunt resistor connected in series with the switching element, where such detection value becomes the value of the current actually flowing to the motor.
In the multi-phase motor driving device described above, a circuit for performing failure detection is arranged since the motor drive cannot be normally performed if failure such as short-circuit occurs. Japanese Unexamined Patent Publication No. 2004-357463 and Japanese Unexamined Patent Publication No. 2005-143153 describe such failure detection circuit. In Japanese Unexamined Patent Publication No. 2004-357463, a detection circuit for detecting an abnormality judgment voltage based on the drain voltage and the gate voltage of the FET on the lower side of each phase of the inverter circuit is arranged, where the voltage value of the voltage detected by such detection circuit and a predetermined value are compared, and the control signal output to the FET is stopped if the voltage value is greater than the predetermined value. In Japanese Unexamined Patent Publication No. 2005-143153, the abnormality of the current value flowing to each phase is detected by comparing two current values obtained by actual measurement or estimation such as the actual current value flowing to each phase of the inverter circuit and the estimated current value.